The present invention relates generally to apparatus for controlling energization of an electrical load in response to liquid temperature, and, more specifically, to an electronic implementation of such apparatus in which the liquid temperature sensing element and all electronic circuitry, except for a manually adjustable temperature setpoint control device, if any, are contained within a liquid impervious capsule.
Boilers, storage tanks, recreational and therapeutic hot tubs, and a variety of other equipment containing water or other liquid whose temperature must be controlled typically require a thermostatic control device or aquastat responsive to temperature of the liquid to achieve the necessary control function. The most common form of aquastat comprises a sealed bulb connected through a capillary tube to a chamber partially bounded by a diaphragm which serves as an electromechanical switch actuator, the bulb, capillary tube and diaphragm chamber being filled with a liquid having desired thermal expansion properties. The switch is typically used to control electrical energization to a fuel valve, such as a gas valve, a water or steam vane, or other desired load.
Such aquastat construction is well known, and certain industry standards regarding form, fit and function have been adopted. This is particularly true for the temperature sensing bulb which extends into the tank, pressure vessel or plumbing. Specifically, the bulb may be located in a well which provides integrity of the liquid container against leakage, in which case the bulb must fit snugly within the well to provide good thermal conductivity. Alternatively, the bulb may cooperate with a compression fitting or other suitable fitting in a port in the container to provide integrity against leakage. In either arrangement, industry. standard hardware is designed for a bulb having specific dimensions. The most common standard bulb diameter is 3/8 ths of an inch.
Although the design of such aquastats and related production equipment and methods have been refined over many years, and are well developed, such aquastats have several inherent shortcomings partially attributable to the mechanical nature of the design and the requirement for several manual operations associated with filling and sealing the bulb-capillary tube-diaphragm chamber system. Such shortcomings include unit-to-unit variations in the nominal control point of the manufactured devices. As a result, the control temperature for a particular application may not fall within the range of temperatures to which the aquastat can be calibrated. This results in a relatively high production rejection rate which contributes to unit cost. An additional contributor to cost of the design is its requirement for high content of various metals.
A further shortcoming stems from the requirement that the bulb and the switch apparatus must be connected by a capillary tube. Thus, there is a practical limit to the spacing between the bulb and switch apparatus, and unless the aquastat is specially manufactured for a particular installation, arrangements must be made for accommodating the excess length of capillary tube.
Finally, the diaphragm actuator and electrical switch are relatively large, and typically separated from the sensing bulb location, thereby resulting in limitations in the design of equipment incorporating an aquastat, and complicating aquastat installation.
Electronic aquastats have been proposed to overcome some of the previously noted shortcomings of conventional aquastat design. In an electronic aquastat, the liquid filled temperature sensing bulb is replaced by a thermistor or other device whose electrical characteristics vary with temperature. The thermistor is contained in a capsule having dimensions which are compatible with the mounting hardware for traditional liquid filled bulb temperature sensors. The thermistor is connected by means of a pair of leads to an electronic circuit typically fabricated on a circuit board and contained in a housing to be mounted at a separate location. Such a design offers greater flexibility than the previous designs, in that the connecting electrical leads may be substantially any desired length and require less care in handling and routing than a liquid filled capillary tube. Further, the electronic implementation is potentially less expensive, is adapted to more automated to manufacturing processes, and is smaller, thus providing greater equipment design and aquastat installation flexibility. The electronic implementation design is also inherently more accurate, thus providing for improved production yields, as well as performance benefits.
Although the proposed electronically implemented aquastat design offers various advantages, it concurrently introduces certain complications. Also, there remain areas in the design in which further improvements are desired.
One complication arises from the nature of a common system in which an aquastat is used. In particular, such a system employs an electrically operated fuel gas valve which is energized from a thermoelectric generator powered from the heat of a standing pilot flame. The electric power output of the generator is very low, and a special low power consuming gas valve system is required for use therewith. Such a gas valve system is commercially available from Honeywell Inc., with the generator being marketed under the tradename "Powerpile".
A conventional aquastat can be readily incorporated into such a system since it consumes substantially no electric power. Conversely, an electronic aquastat does consume electric power, and because electric power in a thermoelectric generator based system is limited, the electronic circuitry must be designed for very low power consumption. A circuit suitable for use in such an application is shown and described in detail in U.S. Pat. Nos. 4,696,369 and 4,734,658 issued on Sep.1987, and Mar. 29, 1988, respectively, and assigned to the assignee of the present application.
One feature of the proposed electronic aquastat in which further improvement is desired relates to cost. At the present state of development, the proposed electronic aquastat is somewhat more costly to produce than a traditional aquastat based on a liquid filled bulb sensor. A feature which contributes to cost and manufacturing complexity is a requirement for sealing at least portions of the electronics against the effects of moisture. This is an important requirement particularly in hot tub and certain other applications in which high humidity or otherwise moist conditions exist. Also, in many applications, it is necessary to provide electromagnetic shielding, which imposes a further requirement on the housing or packaging for the electronics.
Finally, the proposed electronic aquastat design involves two separate components, each of which must be individually accommodated in the equipment design and the installation process. Obviously, the equipment design and installation process would be simplified with an aquastat incorporated into a single package.